It has become common practice to dress drag type well drilling bits with cutting elements made of polycrystalline diamond compacts, or "PDC." Unlike a roller type drill bit, which primarily crushes the earth formation being drilled, a drag type bit more typically actually cuts or chips the earth formation. Thus, the use of diamond in the cutting elements is especially important in drag type bits in order to increase their life. The polycrystalline diamond material typically is supplied in the form of a relatively thin layer on one face of a substantially larger mounting body. The mounting body is usually post-like in configuration, and formed of a relatively hard material such as sintered tungsten carbide. The diamond layer may be mounted directly on the mounting body, or it may be mounted via an intermediate disc-like carrier, also comprised of sintered tungsten carbide. In any event, the diamond layer is disposed toward one end of the mounting body, the other end of which is mounted in a bore, pocket, or recess in the body of the drilling bit.
The bit body itself may be formed of a tungsten carbide matrix. Traditionally, drag bit bodies have also been made of various forms of steel. One problem which has been associated with the use of PDC type cutting members in such drag bit bodies has been damage to and/or loss of these cutting members. This may occur by cracking and shearing of the stud-like mounting body, which carries the diamond layer, near the outer surface of the bit body. Cutting members may also be lost when the mounting bodies become completely dislodged from the recesses in which they are mounted.
U.S. Pat. No. 4,244,432 discloses one form of prior drag bit. Although the bit has a pin and substructure of metal, it is essentially a tungsten carbide matrix type bit in that it comprises a thick layer of such matrix forming the operating end face and extending inwardly therefrom so that the recesses for mounting of the cutting members, as well as the circulation port system, are all formed of the tungsten carbide matrix. This outer matrix portion of the bit has a stepped configuration which, to a certain extent, provides improved support for the mounting bodies of the cutting members. However, the use of tungsten carbide matrix material for forming any substantial part of a bit body entails a number of disadvantages. In the first place, the tungsten carbide matrix material is per se relatively expensive. Furthermore, while highly wear resistant, this material lacks resiliency and is relatively susceptible to cracking and similar type damage. This last characteristic effectively limits the types of manufacturing procedures which may be utilized in forming matrix type bits. For example, any substantial amount of machining of such bits is highly impractical, and the essential configuration of the matrix body must be achieved by other techniques, essentially analogous to casting. Furthermore, it is extremely difficult to mount the cutting members in the recesses in the matrix bit body with an interference fit without damaging the bit body, the cutting members or both. Therefore, as a practical matter, the mounting bodies of the cutting members must be brazed into the recesses in the bit body. These more complicated manufacturing techniques, which are necessitated by the use of tungsten carbide matrix in the bit body, further increase the cost of the bit. Indeed, successful manufacturing of matrix type bits requires particular skill, expertise, and "art" not typically possessed by the average shop hand. Still another disadvantage of the matrix type bit body is its relatively poor thermal conductivity.
A number of the above disadvantages of matrix type bit bodies could, at least theoretically, be ameliorated by the use of a generally non-frangible metallic material, such as a suitable steel, for use in forming the bulk of the bit body. However, although there have been numerous efforts, beginning in the early to mid 1970's, to develop steel body drag bits with PDC cutting members, such efforts have not been entirely successful and, in particular, have not provided an adequate solution to the problem of damage and/or loss of the cutting members in use. Some of the earliest steel body PDC bits included a number of bores each with a concentric counterbore, the pairs of bores being located at various positions about the operating or cutting face of the bit body. The innermost bore of each pair provided the recess for mounting of the mounting body of the cutting member, whereas the larger but shallower outer bore provided access to the entirety of the diamond cutting face, theoretically for cooling and cleaning by the drilling mud. However, it was found that the mounting bodies of the cutting members on such bits did tend to crack or shear off as described hereinabove. Furthermore, the cooling and cleaning of the cutting faces by the drilling mud with such arrangements was less than satisfactory.
U.S. Pat. Nos. 4,323,130 and 4,265,324 illustrate efforts to improve upon the last-mentioned design by providing eccentric, rather than concentric, counterbores. Although these concentric arrangements provided some additional support for the mounting body of the cutting member in the area opposite the cutting face, still further improvements were desired. Additionally, the concentric counterbore scheme did not significantly improve the cooling and cleaning characteristics of the more basic concentric counterbore arrangement.